There exists a need for imparting dew resistance to transparent substrates such as windshields, lenses, goggles, and windows, and reflective substrates such as mirrors and retroreflective traffic signs. While retroreflective traffic signs currently provide optimum levels of headlight reflectivity to motorists, accumulation of dewdrops on the surface of the retroreflective sign can result in potentially catastrophic “blackouts” in which the signs are ineffective in providing vital information to motorists. This problem has been described by John A. Wagner in the Florida Sate Department of Transportation Report “HPR Research Study M29-82”, October 1989. In certain parts of the world, the climate is such that moisture from the atmosphere readily condenses onto surfaces when the temperature of the surface drops below the dew point, the temperature at which the air is fully saturated with water vapor and below which precipitation of water in the form of “dew” occurs. When formed on the surface of mirrors and retroreflective surfaces, these dewdrops scatter the incident light, resulting in the loss of reflectivity or “blackouts”.
One method of preventing condensation and the formation of dewdrops is to heat the surface of the substrate to a temperature above the dew point. U.S. Pat. No. 5,087,508 describes the use of phase change materials in a thermal reservoir located behind the outer layer of a display sign. The phase change material undergoes at least one phase change, e.g., from liquid to solid state or from one crystalline state to another, between about −20° C. and about 40° C. During periods of falling ambient temperature, the thermal reservoir will yield heat, thereby warming the outer layer of the display sign. European Patent Application 155,572 describes a device for preventing the formation of dew and frost on retroreflective road sign carriers in which a thermal radiator is arranged above and in front of the road sign. Neither of these devices provides a complete solution to the problems associated with the formation of dew. The device of U.S. Pat. No. 5,087,508 requires “recharging” of the phase change material at higher temperatures, while the device of EP 155,572 simply minimizes dew formation by minimizing radiative cooling of sign surfaces to the night sky.
Surfactants have been used to obtain anti-fog properties on the surface of polymer films. The surfactants used are generally small molecules or oligomeric in nature, and present in relatively low concentrations. Examples of surfactants used for anti-fog applications in food packaging and greenhouse products include those described in U.S. Pat. Nos. 4,136,072; 4,609,688; 5,451,460; 5,766,772; 5,846,650; and 6,296,694 and EP 1,110,993. In general, the surfactant coatings are susceptible to water washing due to the low concentrations of surface active molecules. In addition, many of the anti-fog films are not dew resistant and exhibit only a modest decrease in surface water contact angles.
Polymeric forms of hydrophilic surface agents have been disclosed as being useful for anti-fog films. U.S. Pat. No. 5,877,254 describes an anti-fog and scratch resistant polyurethane composition that include an isocyanate prepolymer, a hydrophilic polyol and an isocyanate-reactive surfactant. U.S. Pat. No. 4,080,476 describes an anti-fog coating for optical substrates wherein the coating comprises a polymerized monomer of, for example, 2-acrylamido-2-methyl propane sulfonic acid. International Publication WO 99/07789 describes the use of siloxane derivatives of polyetheralcohols as an anti-fog additive to a polyolefin prior to formation of a polyolefin film. Many of the prior art coatings do not provide a consistent long-lasting anti-fog coating. Rather, the anti-fog properties of these coatings fail after repeated washings with water.